diff --git a/README.md b/README.md
index bbf988c69..46167e812 100644
--- a/README.md
+++ b/README.md
@@ -87,6 +87,15 @@ $response = $operatorClient->DeleteNamespace(
 > that change the behavior of the entire PHP worker.  
 > It's recommended to disable deprecated behavior.
 
+#### Update headers
+
+Client interceptors can supply update headers with `Temporal\Interceptor\Header::fromValues()`.
+For updates and the update portion of `updateWithStart()`, PHP header values use the client's
+configured data converter unless the header already has an explicitly supplied converter.
+Prebuilt `Payload` entries supplied through `Header::fromPayloadCollection()` or `withValue()`
+are forwarded unchanged, even when the same header also contains PHP values. Prepare those
+payloads with any application-specific encoding they require; they are not decoded or re-encoded.
+
 ## Testing
 
 The PHP SDK includes a toolkit for testing Workflows.
diff --git a/psalm-baseline.xml b/psalm-baseline.xml
index 57d72366a..ad7cd2897 100644
--- a/psalm-baseline.xml
+++ b/psalm-baseline.xml
@@ -374,9 +374,6 @@
       <code><![CDATA[getStartWorkflow]]></code>
       <code><![CDATA[getUpdateWorkflow]]></code>
     </PossiblyNullReference>
-    <UndefinedInterfaceMethod>
-      <code><![CDATA[toHeader]]></code>
-    </UndefinedInterfaceMethod>
   </file>
   <file src="src/Internal/Client/WorkflowStub.php">
     <ArgumentTypeCoercion>
@@ -411,9 +408,6 @@
       <code><![CDATA[hasResult]]></code>
       <code><![CDATA[toProtoWorkflowExecution]]></code>
     </PossiblyNullReference>
-    <UndefinedInterfaceMethod>
-      <code><![CDATA[toHeader]]></code>
-    </UndefinedInterfaceMethod>
   </file>
   <file src="src/Internal/Declaration/Dispatcher/AutowiredPayloads.php">
     <PossiblyInvalidArgument>
diff --git a/src/Internal/Client/WorkflowStarter.php b/src/Internal/Client/WorkflowStarter.php
index 7e08e4a3b..0b29ff53d 100644
--- a/src/Internal/Client/WorkflowStarter.php
+++ b/src/Internal/Client/WorkflowStarter.php
@@ -175,7 +175,12 @@ function (UpdateWithStartInput $input): UpdateWithStartOutput {
                 $i = new \Temporal\Api\Update\V1\Input();
                 $i->setName($input->updateInput->updateName);
                 $input->updateInput->arguments->isEmpty() or $i->setArgs($input->updateInput->arguments->toPayloads());
-                $input->updateInput->header->isEmpty() or $i->setHeader($input->updateInput->header->toHeader());
+                $header = $input->updateInput->header;
+                \assert($header instanceof Header);
+                if ($header->getDataConverter() === null) {
+                    $header->setDataConverter($this->converter);
+                }
+                $header->isEmpty() or $i->setHeader($header->toHeader());
                 $r->setInput($i);
 
                 $ops = [
diff --git a/src/Internal/Client/WorkflowStub.php b/src/Internal/Client/WorkflowStub.php
index fb4d7e259..aaf70edf8 100644
--- a/src/Internal/Client/WorkflowStub.php
+++ b/src/Internal/Client/WorkflowStub.php
@@ -314,7 +314,12 @@ static function (
                 $input->arguments->setDataConverter($converter);
                 $input->arguments->setSerializationContext($context);
                 $input->arguments->isEmpty() or $i->setArgs($input->arguments->toPayloads());
-                $input->header->isEmpty() or $i->setHeader($input->header->toHeader());
+                $header = $input->header;
+                \assert($header instanceof Header);
+                if ($header->getDataConverter() === null) {
+                    $header->setDataConverter($converter);
+                }
+                $header->isEmpty() or $i->setHeader($header->toHeader());
                 $r->setInput($i);
 
                 try {
diff --git a/tests/Unit/Client/UpdateHeaderTestCase.php b/tests/Unit/Client/UpdateHeaderTestCase.php
new file mode 100644
index 000000000..b4d581561
--- /dev/null
+++ b/tests/Unit/Client/UpdateHeaderTestCase.php
@@ -0,0 +1,118 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Temporal\Tests\Unit\Client;
+
+use PHPUnit\Framework\Attributes\DataProvider;
+use Temporal\Api\Common\V1\Payload;
+use Temporal\Api\Update\V1\UpdateRef;
+use Temporal\Api\Workflowservice\V1\ExecuteMultiOperationRequest;
+use Temporal\Api\Workflowservice\V1\ExecuteMultiOperationResponse;
+use Temporal\Api\Workflowservice\V1\ExecuteMultiOperationResponse\Response;
+use Temporal\Api\Workflowservice\V1\StartWorkflowExecutionResponse;
+use Temporal\Api\Workflowservice\V1\UpdateWorkflowExecutionRequest;
+use Temporal\Api\Workflowservice\V1\UpdateWorkflowExecutionResponse;
+use Temporal\Client\GRPC\Context;
+use Temporal\Client\GRPC\ServiceClientInterface;
+use Temporal\Client\WorkflowClient;
+use Temporal\DataConverter\DataConverterInterface;
+use Temporal\Interceptor\Header;
+use Temporal\Interceptor\SimplePipelineProvider;
+use Temporal\Interceptor\WorkflowClient\UpdateInput;
+use Temporal\Interceptor\WorkflowClient\UpdateWithStartInput;
+use Temporal\Interceptor\WorkflowClientCallsInterceptor;
+use Temporal\Tests\Unit\AbstractUnit;
+
+final class UpdateHeaderTestCase extends AbstractUnit
+{
+    /**
+     * @return iterable<string, array{bool, bool}>
+     */
+    public static function headers(): iterable
+    {
+        yield 'update, client converter' => [false, false];
+        yield 'update with start, client converter' => [true, false];
+        yield 'update, explicit converter' => [false, true];
+        yield 'update with start, explicit converter' => [true, true];
+    }
+
+    #[DataProvider('headers')]
+    public function testHeaderConversion(bool $withStart, bool $explicitConverter): void
+    {
+        $converter = $this->createMock(DataConverterInterface::class);
+        $headerConverter = $explicitConverter ? $this->createMock(DataConverterInterface::class) : $converter;
+        $encoded = new Payload(['metadata' => ['encoding' => 'custom/header'], 'data' => 'converted']);
+        $raw = new Payload(['metadata' => ['encoding' => 'custom/raw'], 'data' => "\x00raw\xff"]);
+        $rawBytes = $raw->serializeToString();
+
+        $headerConverter->expects(self::once())->method('toPayload')->with('value')->willReturn($encoded);
+        $headerConverter->expects(self::never())->method('fromPayload');
+        if ($explicitConverter) {
+            $converter->expects(self::never())->method('toPayload');
+            $converter->expects(self::never())->method('fromPayload');
+        }
+        $header = $explicitConverter
+            ? Header::fromPayloadCollection(['raw' => $raw], $headerConverter)->withValue('value', 'value')
+            : Header::fromValues(['value' => 'value'])->withValue('raw', $raw);
+
+        $interceptor = $this->createMock(WorkflowClientCallsInterceptor::class);
+        $interceptor->method('update')->willReturnCallback(
+            static fn(UpdateInput $input, callable $next) => $next($input->with(header: $header)),
+        );
+        $interceptor->method('updateWithStart')->willReturnCallback(
+            static fn(UpdateWithStartInput $input, callable $next) => $next(
+                $input->with(updateInput: $input->updateInput->with(header: $header)),
+            ),
+        );
+
+        $service = $this->createMock(ServiceClientInterface::class);
+        $service->method('getContext')->willReturn(Context::default());
+        $service->method('withContext')->willReturnSelf();
+        $response = (new UpdateWorkflowExecutionResponse())->setUpdateRef(
+            (new UpdateRef())->setUpdateId('update-id'),
+        );
+        $checkHeader = static function (UpdateWorkflowExecutionRequest $request) use ($encoded, $rawBytes): void {
+            $fields = $request->getRequest()->getInput()->getHeader()->getFields();
+            self::assertCount(2, $fields);
+            self::assertSame($encoded->serializeToString(), $fields['value']->serializeToString());
+            self::assertSame($rawBytes, $fields['raw']->serializeToString());
+        };
+        if ($withStart) {
+            $service->expects(self::once())->method('ExecuteMultiOperation')->willReturnCallback(
+                static function (ExecuteMultiOperationRequest $request) use ($checkHeader, $response) {
+                    $checkHeader($request->getOperations()[1]->getUpdateWorkflow());
+
+                    return (new ExecuteMultiOperationResponse())->setResponses([
+                        (new Response())->setStartWorkflow(
+                            (new StartWorkflowExecutionResponse())->setRunId('run-id'),
+                        ),
+                        (new Response())->setUpdateWorkflow($response),
+                    ]);
+                },
+            );
+        } else {
+            $service->expects(self::once())->method('UpdateWorkflowExecution')->willReturnCallback(
+                static function (UpdateWorkflowExecutionRequest $request) use ($checkHeader, $response) {
+                    $checkHeader($request);
+
+                    return $response;
+                },
+            );
+        }
+        $client = new WorkflowClient(
+            $service,
+            converter: $converter,
+            interceptorProvider: new SimplePipelineProvider([$interceptor]),
+        );
+
+        if ($withStart) {
+            $client->updateWithStart($client->newUntypedWorkflowStub('workflow'), 'update');
+        } else {
+            $client->newUntypedRunningWorkflowStub('workflow-id')->startUpdate('update');
+        }
+
+        self::assertSame($headerConverter, $header->getDataConverter());
+        self::assertSame($rawBytes, $raw->serializeToString());
+    }
+}